1. Field of the Invention
The present invention relates to a novel low-silica zeolite of an SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of 1.9-2.1 having extraordinary high heat resistance. The present invention relates also to a process for industrial production, and application fields thereof.
The novel low-silica zeolite of an SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of 1.9-2.1 having extraordinary high heat resistance retains the high heat resistance even after ion-exchange with various ions. This low-silica zeolite exhibits extremely high performance, for example, as an adsorption zeolite in separating and concentrating oxygen from an oxygen-nitrogen gas mixture by adsorption, or as a CO.sub.2 adsorbent.
2. Description of the Related Art
The low-silica zeolite of an SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of 1.9-2.1 (hereinafter referred to as "LSX") is known to exhibit high performance as an adsorbent base material for oxygen production, or an adsorbent for CO.sub.2 gas.
However, no technique has been established for industrial production of LSX, and the adsorbent prepared by ion exchange of an LSX prepared by a known laboratory technique is inferior in heat resistance. Therefore, the LSX has not been practicalized industrially.
The performance of conventional LSX exchanged with lithium ion was evaluated in U.S. Pat. No. 3,140,933, JP-B-5-25527, and U.S. Pat. No. 5,268,023. The performance of conventional LSXs exchanged with calcium ion is shown in JP-A-61-254247, JP-A-6-23264, and U.S. Pat. No. 5,454,857.
However, the known LSXs are prepared in a small amount over several days by a laboratory synthesis technique, and the production cannot be practiced industrially. Moreover, the laboratory LSXs are not sufficient in heat resistance.
The laboratory methods for preparation of LSX of the SiO.sub.2 /Al.sub.2 O.sub.2 molar ratio of 1.9-2.1 are disclosed by prior art documents as below:
GB 1,580,928 (corresponding to JP-A-53-8400) discloses a method in which a mixture containing sodium, potassium, aluminate, and silicate is crystallized at a temperature lower than 50.degree. C., or is aged at a temperature lower than 50.degree. C. and then crystallized at a temperature of 60-100.degree. C. This method requires a time of 50 hours or more substantially for preparation of high-purity LSX, which is not suitable for industrial production. The resulting LSX is not satisfactory in heat resistance.
GB 1,580,928 (JP-A-53-8400) investigated in detail the method disclosed by East Germany Patent 43221. However, the resulting LSX also had low water adsorption capacity, and had a low purity.
The above patents cover wide ranges of conditions for preparation of LSX. However, the ranges include regions where LSX cannot be produced.
The inventors of GB 1,580,928 (JP-A-53-8400) presented later a scientific paper (Zeolite, 1987, Vol.7, p.451-457) to disclose the synthesis of LSX in detail. In that paper, high-purity LSX (97% or higher) was obtained by use of a sealed plastic vessel. However, heat resistance was not improved by this method. The synthesis in that document was conducted in a small scale by standing in an oven during the steps of from aging to crystallization, which is not applicable to industrial production.
U.S. Pat. No. 4,859,217 (corresponding to JP-B-5-25527) discloses a method in which a mixture containing sodium, potassium, and aluminate is mixed with another mixture containing silicate at a low temperature of 4-12.degree. C., the mixture is allowed to gel, and the formed gel is aged at 36.degree. C. and crystallized at an elevated temperature of 70.degree. C.
The above patents describe that the gelation takes two to three days, and application of excessive mechanical energy should be avoided.
Even at the time (Application date of U.S. Pat. No. 4,859,217 (Jun. 30, 1987)), the synthesis of LSX takes long time without application of mechanical energy, namely stirring, and the resulting LSX itself is less heat-resistant.
Alternatively, U.S. Pat. No. 4,603,040 (corresponding to JP-A-61-222919) discloses preparation of LSX from kaolin as the alumina and silica source with stirring. In this method, however, the LSX content is no more than about 60% of all zeolite even after the reaction for 100 hours or more, with 10% or more of A-type zeolite produced as a byproduct, and the adverse effect of stirring is reconfirmed for high-purity LSX production. Moreover, the formed "macroscopic condensate" has a particle diameter exceeding 50 .mu.m. Therefore, the resulting LSX, after ion exchange, does not give sufficient nitrogen adsorption capacity when it is used for nitrogen adsorption from air in high-purity oxygen production by a pressure-swing adsorption (hereinafter referred to as "PSA"), being not suitable as the base low-silica zeolite for PSA gas separation. The LSX is not useful also for CO.sub.2 gas adsorption for the same reason.
At the moment, the low-silica zeolite (LSX) is believed to be producible only by reaction for a long time with still-standing. No disclosure is found on industrial process for LSX, and improvement of the heat resistance of the industrial LSX. The high performance of the adsorbent employing the LSX as the base zeolite is confirmed in laboratory only, and has not been realized industrially.
On the other hand, industrial production of oxygen by the PSA process is practiced in iron production with a blast furnace, glass production in a fusion furnace, bleaching, fermentation, and so forth by use of an adsorbent in an amount of from tons to several tens of tons in one batch. Therefore, the term "industrial production" herein means production of several tons or more of zeolite in one batch, not production in several kilograms.
Known LSX-based adsorbents are produced by ion exchange of LSX with lithium cation; alkaline earth metal cation such as calcium cation and strontium cation; or composite cation of lithium cation with another cation such as alkaline earth metal cation. They are produced in a laboratory, and is not heat-resistant (e.g., U.S. Pat. No. 5,152,813).
U.S. Pat. No. 3,140,923 discloses that faujasite exchanged with lithium ion exhibits high performance in nitrogen adsorption, higher at a higher lithium ion exchange ratio, and the faujasites of an SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of up to 2.0 are useful therefore. This USP does not mention the heat resistance of the faujasite employed.
The faujasite exchanged with lithium ion at a higher exchange ratio was evaluated further by U.S. Pat. No. 4,859,217 (corresponding to JP-B-5-25527) and U.S. Pat. No. 5,268,023, and the properties are shown in detail. However, the LSX was prepared by a conventional method in the disclosures, and was less heat-resistant.
The inventor of the above U.S. Pat. No. 4,859,217, Chien C. Chao, indicated the low heat resistance of lithium-exchanged faujasite in U.S. Pat. No. 5,174,979, and reported the improvement of the heat resistance of the faujasite by mixed ion exchange with lithium ion and alkaline earth metal ion. The faujasite obtained by exchange with mixed ions of lithium and an alkaline earth metal, although the heat resistance thereof is improved, has a poor adsorption ability, especially at a low temperature, so that the improvement of the heat resistance is of no value.
Zeolite adsorbents derived from LSX by exchange with cation of an alkaline earth metal such as calcium and strontium are disclosed in JP-A-61-254247, U.S. Pat. No. 5,173,462, U.S. Pat. No. 5,454,857, and so forth. However, they are prepared from conventional laboratory LSX as disclosed by GB 1,580,928 (corresponding to JP-A-53-8400) and other patents, and is not sufficient in heat resistance like the aforementioned lithium-exchanged one.
The LSX, having potentiality of high performance, has not been used in industrial gas separation by PSA because of difficulty in industrial LSX production and insufficient heat resistance of the conventional LSX.